I pretty much work the same way as you, OP. Most of my PS3 library is exclusives, because I got my 360 first.
I do it mostly because if its a franchise I started on the Xbox, I want to continue it on the same console. This ended up coming to bite me in the ass when Dead Space 2 came out, but I don't have the Move so I figure its no real loss.
If its a new franchise, however, I usually put time into thinking which console I'd rather play it on.
I bought Final Fantasy XIII for the PS3 because it was made for PS3 and playing it on the Xbox would just feel... wrong.
I also save all fighting games for the PS3, but that's more because I've got a friend who's WAY into fighting games but he only has a PS3.
Multi-platform shooters will probably always go to the Xbox, though, because I just can't stand the PS3's triggers (Portal 2 being the only exception).
Multiplayer-heavy games usually go to the Xbox too, because my friends list is bigger on the Xbox than the PS3.
I try not to buy any games for my PC, because it's a hunk of crap. Only MMOs and TF2.